In past years, great progress has been made in developing acid-resistant and etch-resistant clearcoats for the OEM finishing of automobiles. More recently, there is now an increasing desire in the automotive industry for scratch-resistant clearcoats which at the same time retain the level attained hitherto in terms of their other properties.
At present, however, there are different test methods for the quantitative assessment of the scratch resistance of a coating, examples being testing by means of the BASF brush test, by means of the washing brush unit from the company AMTEC, or various test methods of automakers and others. A disadvantage, however, is that it is not possible in every case to correlate the individual test results. In other words, the test results for one and the same coating may have very different outcomes result depending on the test method chosen, and passing one scratch resistance test does not, under certain circumstances, permit conclusions to be drawn about the behavior of that coating in a different scratch test.
There is, therefore, a desire for a method of quantitatively assessing the scratch resistance which enables reliable statements to be made about the scratch resistance of the coating from just one examination of the sample. In particular, the result of this examination should permit reliable conclusions to be drawn about the scratch resistance of the coating in the various abovementioned scratch resistance tests.
The literature, indeed, has already described a number of investigations relating to the physical processes taking place during the production of scratches, and correlations derived therefrom, between the scratch resistance and other physical parameters of the coating. A contemporary review of various literature relating to scratch-resistant coatings can be found, for example, in J. L. Courter, 23.sup.rd Annual International Waterborne, High-Solids and Powder Coatings Symposium, New Orleans 1996.
Furthermore, for example, the article by S. Sano et al., "Relationship between Viscoelastic Property and Scratch Resistance of Top-Coat Clear Film," Toso Kagaku 1994, 29 (12), pages 475-480, uses a washing brush test to determine the scratch resistance of different, heat-curing melamine/acrylate or isocyanate/acrylate systems and correlates the scratch resistance found with viscoelastic properties of the coating.
From the test results described in that article, the authors conclude that coatings exhibit good scratch resistance when either the so-called "inter-crosslinking molecular weight" is below 500 or when the glass transition temperature is 15.degree. C. or less. It is necessary, however, in the case of clearcoat films in the automotive sector, for the glass transition temperature to be above 15.degree. C. in order to achieve sufficient hardness of the coatings. The improvement in the scratch resistance by increasing the number of crosslinking points often leads in practice, moreover, to diverse problems, such as, for example, an inadequate storage stability and an often incomplete reaction of all crosslinking sites.
In the article by M. Rosler, E. Klinke and G. Kunz in Farbe+Lack, Volume 10, 1994, pages 837-843, too, the scratch resistance of various coatings is investigated by means of different test methods. The article found that, under a given load, hard coatings exhibit greater damage and thus lower scratch resistance than soft coatings.
Furthermore, in the conference report of B. V. Gregorovich and P. J. McGonical, Proceedings of the Advanced Coatings Technology Conference, Illinois, USA, Nov. 3-5, 1992, pages 121-125, it is found that increasing the plastic nature (toughness) of coatings improves the scratch resistance, owing to the improved plastic flow (scratch healing), although limits are imposed on the increase in plastic nature by the other properties of the coating.
Furthermore, P. Betz and A. Bartelt in Progress in Organic Coatings, 22 (1993), pages 27-37, disclose various methods of determining the scratch resistance of coatings. That article makes reference, furthermore, to the fact that the scratch resistance of coatings is influenced not only by the glass transition temperature but also, for example, by the homogeneity of the network.
That article proposes increasing the scratch resistance of clearcoat coatings by incorporating siloxane macromonomers, since these siloxane macromonomers lead to increased homogeneity of the clearcoat surface and, above 60.degree. C., to an improved plastic flow.
The correlation between storage modulus and crosslinking density, finally, is known, for example, from Loren W. Hill, Journal of Coatings Technology, Vol. 64, No. 808, May 1992, pages 29 to 41. However, that article contains no statements or indications as to how scratch-resistant coatings can be obtained.
EP-A-540 884, furthermore, discloses a process for producing multicoat finishes, especially in the automotive sector, using free-radically and/or cationically polymerizable, silicone-containing clearcoats, in which the application of the clearcoat takes place under illumination with light having a wavelength of more than 550 nm or with exclusion of light, and in which, subsequently, the clearcoat film is cured by means of high-energy radiation. The surfaces obtained in this way are said to have good optical characteristics and a good scratch resistance. Further details on the level of the scratch resistance, and details of how the scratch resistance was determined, are, however, not contained in EP-A-540 884.
Finally, EP-A-568 967 also discloses a process for producing multicoat finishes, especially in the automotive sector, using radiation-curable clearcoats. According to EP-A-568 967, however, it is essential to the invention that in order to obtain clearcoat films having a high optical quality first of all a heat-curing clearcoat and thereafter a radiation-curable clearcoat is applied.